1. Field of the Invention
This invention relates generally to a video tape recorder that can accommodate different kinds of recording tape and, more particularly, to an automatic recording mode detecting circuit for use in such video tape recorder that detects the recording mode for proper playback.
2. Description of the Background
A video tape recorder (VTR) for home-use can usually record a video signal according to a standard picture mode and a high-quality picture mode. The standard picture mode of recording for the home video tape recorder is provided in accordance with the standards that were determined in the early stages of design of video tape recorders. The high-quality picture mode, on the other hand, is provided in accordance with standards developed in light of recent improvements in the performance of the magnetic tape and the magnetic heads used in such VTR's. In the original design of VTR's, the only high-quality picture mode that was proposed required an expensive tape medium.
In the standard picture mode and the high-quality picture mode, the carrier frequencies of the FM luminance signals Y.sub.NFM and Y.sub.WFM, corresponding respectively to the standard and high-quality modes, are different, and this difference is shown in Figs. 1A and IB. Because of this difference the two modes are not completely compatible. Accordingly, the video signal must be reproduced in the same mode as the recording mode. This requires a circuit for detecting the particular recording mode. For example, U.S. Pat. No. 4,811,122 describes an automatic recording mode detecting circuit, in which the carrier frequency of the reproduced FM luminance signal is detected, or the mean DC level of an FM-demodulated reproduced luminance signal is detected, and one or the other is used to automatically determine the recording mode without requiring the user to set a switch.
Nevertheless, the above-described recording mode detecting circuit requires a plurality of bandpass filters, through which the carriers of respective modes can pass, and the circuit scale is considerably increased so that it cannot be fabricated as an integrated circuit. Further, detection of a mean DC level by using an analog circuit cannot be done without difficulty. This raises the substantial possibility that a detection error will occur.
Furthermore, there is a substantial risk that a detection error will occur in the circuit that detects the level of the FM signal extracted by the above-described bandpass filters, due to a side-band component of the FM signal.
Just as importantly, however, is the fact that the aforenoted circuit cannot be fabricated as an integrated circuit, such as a complementary metal oxide semiconductor (C-MOS) or the like, and also the fact that the automatic recording mode detecting circuit cannot be driven by a low-power circuit.